In wireless communication systems, a signal may be sent at a certain frequency within specified parameters, in what is called a transmission path. Recent developments have enabled the simultaneous transmission of multiple signals over a single transmission path. One of these methods of simultaneous transmission is referred to as Frequency Division Multiplexing (FDM). In FDM, the transmission path is divided into sub-channels. Information (e.g. voice, video, audio, text, data, etc. . . . ) is modulated and transmitted over the sub-channels at different sub-carrier frequencies.
A particular type of FDM is Orthogonal Frequency Division Multiplexing (OFDM). In OFDM technology, the sub-carrier frequencies are spaced apart by precise frequency differences. An advantage of OFDM technology is that it is generally able to overcome multiple path effects. Another advantage of OFDM technology is that it is typically able to transmit and receive large amounts of information. A further advantage is that by using multiple transmitting antennas and multiple receiving antennas in an OFDM system, it is possible to increase the capacity of transmitted and received data while generally using the same amount of bandwidth as in a system with one transmitting and one receiving antenna. Because of these advantages, much research has been performed to advance OFDM technology.
OFDM technology is typically divided into two categories: Single-Input, Single-Output (SISO); and Multi-Input, Multi-Output (MIMO). SISO utilizes a single transmitting antenna to transmit signals and a single receiving antenna to receive the signals. MIMO uses multiple transmitting antennas and multiple receiving antennas.
In typical communication systems, a preamble, at the beginning of each data transmission, is usually added as a prefix to the data symbols. The data symbols, of course, include the useful data or information (e.g., voice, data, video, etc. . . . ), which is meant to be transmitted to a remote location. The preamble is used to provide information such as frequency tuning, synchronization, and channel parameter estimation. The receiver for an OFDM system in the acquisition mode uses the information in the preamble to perform time synchronization, frequency offset estimation and correction, and channel estimation. Sampling frequency offset may create Inter Carrier Interference (ICI), phase rotation, amplitude distortion and loss in synchronization.
Data in the preamble presents a sampling frequency, which is used to determine if there is a frequency offset between the transmitter and the receiver. The sampling frequency is offset in almost all systems. The transmitter and receiver each have digital clocks with oscillators and those can never be exactly synchronized. The effect of the offset gets worse over time.
A receiving system can monitor and adjust the sampling frequency in one of two ways: using an open loop system or a closed loop system. An open loop system reads the sampling frequency, monitors the offset, and performs appropriate phase rotations and timing adjustment, as the signal passes through the receiver. A closed loop system generates an error signal proportional to the sampling frequency offset, and performs appropriate phase rotations and timing adjustments.
An open loop system is adequate for indoor wireless or fixed wireless applications. But certain signals, such as broadcast signals such as streaming video, change over time and typically need to be continuously monitored. These signals require a closed loop system.
Both open loop and closed loop systems exist for SISO OFDM systems. However, for MIMO OFDM systems, so far as is known, neither open loop nor a closed loop system exists for monitoring and adjusting the sampling frequency offset. In fact, no method or system is known to exist for a MIMO OFDM system that is capable of providing corrections to the sampling frequency offset as part of the signal synchronization.
Accordingly, there is a need for a method or system that is capable of detecting, correcting, and controlling the sampling frequency offset as part of signal synchronization in a MIMO OFDM system.